And I can tell you of a number of large systems which are web based that have a problem with that! In one case I know of you put an apostrophe in an email address and the Carp output will then give you a clue to the 'backdoor' super-user type access into the system without having to authenticate.
All for the sake of the most simple untainting. Whether it is a valid email address or not - an apostrophe is not permitted in an email address! Fortunately the data on the system is not extremely valuable. It is an email autoresponder system which handles a lot of marketting email. But then again I did wonder how an address of mine that was in somebody's newsletter list suddenly started getting spam. I suspect the spammers have been in through the back door and downloaded all the lists out of the system.